Concrete is a mixture of paste and aggregates. The paste is composed of cement and water. The most common cement is Portland cement, although other cementous materials may be used, such as fly ash, ground slag, and silica fume. The aggregates may include both fine and course aggregates, such as sand and rocks, respectively. Freshly mixed, uncured concrete is plastic. It can be molded or formed into any shape, which becomes strong and durable when hardened. Careful proportioning and mixing of the ingredients is key to producing strong, durable concrete. A concrete mixture with insufficient paste to fill the voids between the aggregates will be difficult to place, and will produce rough, honey combed surfaces and porous concrete. A mixture with excessive paste is easy to place and produces a smooth surface, but produces concrete that is likely to crack. Thus, the desired workability for the fresh concrete and the required durability and strength for the hardened concrete depends on properly proportioning the ingredients. Generally, a concrete mixture contains approximately 10%-15% cement, 60%-75% aggregates, and 15%-20% water, by volume. Air may also be introduced into the mixture at 5%-8% by volume.
The quality of the paste determines the character of the concrete. The paste strength depends on the water to cement ratio. Ideally, the water to cement ratio is lowered as much as possible to produce high quality concrete, without sacrificing the workability of the uncured mixture.
Concrete can be produced at a stationary plant, with a ready-mix truck, or a volumetric mixing system. A stationary plant includes all the storage, mixing and delivery components assembled at the job site to produce concrete for extended periods of time. Ready-mix refers to concrete that is from a central stationary plant, wherein the aggregate, cement and water are mixed in a rotating barrel on a truck which delivers the slurry to the job site, rather than being mixed at the job site. Ready mix is advantageous for small jobs when intermittent placing of concrete is required. In volumetric systems, the aggregate, cement and water are stored in separate bins or compartments on a truck, and then mixed together at the job site in a mixing boot on the end of the truck.
In conventional volumetric mixing systems, the sand and rock aggregates pass through a pair of gates for discharge onto a conveyor belt. The volume of the respective aggregates can be controlled by adjusting the gate opening to achieve the desired concrete mix design. The truck also includes a cement bin with an auger that discharges the cement into the aggregate mixture. These solid ingredients are measured in a volumetric manner to regulate the mixed design. For example, the volume of each ingredient can be calculated by the size of the respective gate opening, the speed of the cement auger, and the speed of the conveyor. However, if the sand, aggregate or cement bridges in their bin so that delivery to the conveyor is not complete, the desired mix ratio is not achieved. Therefore, the operator normally must watch the slurry discharged from the mixing boot to assure consistent slump. If a change in slump is noticed, the operator must determine the cause and solve the problem, such as breaking up the bridged ingredient. Such a fix often requires the mixer to be shut down temporarily, thus slowing down the whole concrete operation.
Even when everything is operating correctly such that the volume measurements are relatively accurate, new standards requiring greater accuracy cannot be achieved with conventional volumetric measurement of the aggregates and cement.
Therefore, a primary objective of the present invention is the provision of an improved volumetric concrete mixing system and method using load cells to perform a weight loss function for aggregates and cement.
Another objective of the present invention is the provision of an improved volumetric concrete truck having aggregate bins and a cement bin which are independently mounted from one another for independent weight measurements of the bin contents.
Yet another objective of the present invention is the provision of an improved volumetric concrete truck having a programmable control to receive data corresponding to ingredient weight measurements and adjusting ingredient delivery to achieve a desired concrete mix specification.
Still another objective of the present invention is the provision of an improved volumetric concrete mixing system and method which automatically and accurately determines the weight of materials delivered from the dry ingredient storage bins to the mixing boot.
A further objective of the present invention is the provision of an improved volumetric concrete mixing system which automatically adjusts the delivery of aggregate and cement to maintain a desired mix ratio.
Another objective of the present invention is the provision of an improved volumetric concrete mixing system which eliminates or minimizes the need for an operator to monitor the mix slurry.
Still another objective of the present invention is the provision of an improved volumetric concrete mixing system and method which quickly and easily allows for a change of mix ratios.
These and other objectives will become apparent from the following description of the invention.